CN109804513B

CN109804513B - Waveguide structure

Info

Publication number: CN109804513B
Application number: CN201780060157.5A
Authority: CN
Inventors: S·戴维斯; M·科尔立; S·法维尔
Original assignee: Langmeitong Technology Uk Ltd
Current assignee: Langmeitong Technology Uk Ltd
Priority date: 2016-09-29
Filing date: 2017-09-29
Publication date: 2021-03-30
Anticipated expiration: 2037-09-29
Also published as: GB2554460A; CN109804513A; DE112017004914B4; JP6696051B2; GB2556995A; DE112017004914T5; US10746922B2; WO2018060729A1; GB2556995B; JP2019530978A; GB201715826D0; GB201616562D0; US20190369328A1

Abstract

A waveguide structure, including a waveguide having a thermally controllable portion, and a method of making the structure. The waveguide structure includes a plurality of layers. These layers comprise in sequence: a substrate (306), a sacrificial layer (305), a lower cladding layer (303), a waveguide core layer (302), and an upper cladding layer (301). The lower cladding, the waveguide core layer, and the upper cladding form a waveguide having a waveguide core. The waveguide structure has a continuous via (307) passing through the upper cladding layer, the waveguide core layer and the lower cladding layer and extending parallel to the waveguide ridge (304) along substantially the entire length of the thermally controllable portion. The waveguide structure also has an adiabatic region (308) in the sacrificial layer that extends along an entire length of the thermally controllable portion at least from the via hole to outside the waveguide ridge. The sacrificial layer includes a sacrificial material outside of the insulating region and an insulating gap (308) or insulating material separating the lower cladding and the substrate within the insulating region. The structure is fabricated by providing a wet etch to the sacrificial layer through the via to remove material from at least the adiabatic regions.

Description

Waveguide structure

Technical Field

The present invention relates to a waveguide structure. In particular, the present invention relates to an improved waveguide structure, including a waveguide having a thermally controllable portion, and methods of making the same.

Background

Where the term "light" is used, this refers generally to electromagnetic radiation, and not specifically to visible light. Where the term "laser" is used, this refers to a semiconductor laser unless otherwise indicated.

Thermally tuned semiconductor lasers (e.g., distributed bragg reflectors, DBRs, lasers) are being developed to improve linewidth performance compared to known electronically tuned lasers. Each type of tuning works by modifying the refractive index of one or more components of the laser (e.g., the reflector) so that the components select different wavelengths.

Electronically tuned lasers provide a high level of optical loss, which increases the laser threshold current and reduces the linewidth. Furthermore, since electronic tuning has a very fast response (on the order of nanoseconds), electronic noise is easily coupled to the laser output.

In contrast, thermal tuning does not significantly increase optical loss, and thus the linewidth reduction is negligible. Furthermore, since the response of thermal tuning is much slower (on the order of tens of microseconds), the laser output is separated from the high frequency noise source. Heat is applied to the waveguide optical core by resistive heater strips disposed on top of or closely parallel to the waveguide ridge. The strips are electrically isolated from the ridges by a passivating dielectric.

A typical electronically tuned laser has a cross-section as shown in fig. 1A. The laser includes a p-cladding layer 101, a waveguide core layer 102, an n-cladding layer 103, and a substrate 104. The p-cladding layer 101 is etched to form a waveguide ridge 105, and electrical means (not shown) for changing the refractive index are connected to the waveguide ridge 105. The region of the waveguide core layer below the waveguide ridge forms the waveguide core.

Figure 1B shows a "buried heterostructure" laser. The laser includes a p-cladding layer 111, a waveguide core layer 112, an n-cladding layer 113, and a substrate 114. Instead of the waveguide ridge 115, the waveguide is formed by a structure 115 in the upper cladding layer and the waveguide core layer, which is isolated by an isolation region 116. The waveguide core of the buried heterostructure laser is formed by portions of the waveguide core layer within the structure 115.

Each laser is a planar structure of a material that dissipates heat well and is intended to extract the heat generated by the diode. However, this means that the power required to cause the necessary temperature change is very large (e.g. 1W for a temperature change of 50-70 ℃) when thermally tuned with this laser design. In order to improve the efficiency of thermal tuning, it is desirable to thermally isolate the waveguide from the support structure. However, the portions of the laser that are not thermally tuned should be in thermal contact with the support structure to enable their temperature to remain constant.

An exemplary known structure for achieving this is shown in fig. 2A and 2B for a ridge waveguide laser, where fig. 2A is a cross-sectional view of the structure along line IIA-IIA in fig. 2B and fig. 2B is a plan view. The laser has an upper p-cladding layer 201, a waveguide core 202, and a lower n-cladding layer 203. The upper p-cladding layer is etched to form waveguide ridge 204. A layer of sacrificial material 205 is located between the lower cladding layer and the substrate 206 and is etched away by a wet etch process to leave an air gap 208 under the portion containing the waveguide ridge. Vias 207 are provided in the upper cladding, waveguide core and lower cladding to allow wet etching to reach the sacrificial material. Obviously, if the via completely surrounds the waveguide, the waveguide will no longer be supported, and therefore a support structure 209 is provided in the via to connect the waveguide to the rest of the substrate.

However, these support structures result in waveguides having non-uniform thermal characteristics-i.e., portions of the waveguide near the support structures will cool more readily than portions away from the support structures. This uneven heating affects the uniform control of the refractive index along the part and degrades the performance of the laser.

Disclosure of Invention

According to a first aspect, there is provided a waveguide structure comprising a waveguide having a thermally controllable portion. The waveguide structure includes a plurality of layers and a ground contact. These layers comprise in sequence: the waveguide comprises a substrate, a sacrificial layer, a lower cladding layer, a waveguide core layer and an upper cladding layer. The lower cladding, the waveguide core layer, and the upper cladding form a waveguide having a waveguide core. The waveguide structure has a continuous via that passes through the upper cladding layer, the waveguide core layer and the lower cladding layer and extends parallel to the waveguide ridge along substantially the entire length of the thermally controllable portion on only one side of the waveguide ridge. The waveguide structure further has an adiabatic region in the sacrificial layer that extends along the entire length of the thermally controllable portion at least from the via to beyond the waveguide ridge. The sacrificial layer includes a sacrificial material outside of the insulating region, and an insulating gap or material separating the lower cladding and the substrate within the insulating region. The ground contact is in electrical contact with the upper cladding on a side of the waveguide core opposite the via; or in electrical contact with the upper cladding adjacent the waveguide core.

According to another aspect, there is provided a tunable laser comprising the waveguide structure of the first aspect.

According to yet another aspect, a method of manufacturing a thermally controlled waveguide is provided. A waveguide structure is provided that includes a plurality of layers and a ground contact. These layers comprise in sequence: the waveguide comprises a substrate, a sacrificial layer, a lower cladding layer, a waveguide core layer and an upper cladding layer. The lower cladding, the waveguide core layer, and the upper cladding form a waveguide having a waveguide core. The waveguide structure has a continuous via that passes through the upper cladding layer, the waveguide core layer and the lower cladding layer and extends parallel to the waveguide ridge along the entire length of the thermally controllable section on only one side of the waveguide ridge. The waveguide structure is arranged such that the ground contact is in electrical contact with the upper cladding on the opposite side of the waveguide core to the through hole, or adjacent the waveguide core. A wet etch is provided through the through-hole to the sacrificial layer to remove material from at least one adiabatic region in the sacrificial layer that extends along the entire length of the thermally controllable portion at least from the through-hole to beyond the waveguide ridge, thereby creating a gap in the adiabatic region that separates the lower cladding from the substrate. The wet etch etches the material of the sacrificial layer and does not etch the material of the substrate and the lower cladding.

Other embodiments of the invention are set forth in claim 2 and the like.

Drawings

FIG. 1 shows a cross-section of a waveguide structure of a typical electronically tuned laser;

FIGS. 2A and 2B show cross-sectional and plan views of a known waveguide structure for a thermally tuned laser;

FIG. 3 illustrates cross-sectional and plan views of an exemplary waveguide structure for a thermally tuned laser;

FIG. 4 shows a stage in the manufacture of the structure of FIG. 3;

FIG. 5 illustrates a typical temperature profile of an exemplary waveguide structure during heating;

FIG. 6 illustrates a cross-section of an exemplary thermally tuned laser;

FIG. 7 illustrates a cross-section of another exemplary thermally tuned laser;

fig. 8 shows a cross-section of an exemplary buried heterostructure laser.

Detailed Description

Another undercut configuration is shown below. This configuration overcomes the limitations of the prior art because it provides a more uniform heat distribution. In addition, the structure is highly tolerant of manufacturing process variations and, in certain embodiments, allows the waveguide structure to be more efficiently grounded than existing undercuts.

Fig. 3A and 3B illustrate exemplary structures, where fig. 3A is a cross-sectional view of the structure along line IIIA-IIIA in fig. 3B, and fig. 3B is a plan view. Similar to the prior art structure of fig. 2, the structure has an upper p-cladding layer 301 and a lower n-cladding layer 303 sandwiched over a waveguide core 302. The upper p-cladding layer is etched to provide waveguide ridges 304. Sacrificial layer 305 is disposed between the lower n-cladding layer and substrate 306. Rather than providing a series of through holes on either side of the waveguide as in known structures, a single through hole 307 is provided on one side of the waveguide. There is no support structure passing through the via and the via extends through the upper cladding layer 301, the waveguide core layer 302 and the lower cladding layer 303 up to the sacrificial layer 305. The through-hole extends parallel to the waveguide ridge along the entire length of the thermally controllable section. An etching fluid is provided into the via to etch the sacrificial layer, which results in the portion containing the waveguide ridge overhanging the air gap 308 in a cantilevered arrangement. Sufficient thermal performance can be obtained as long as the sacrificial material is etched at least beyond the waveguide, as this will result in the waveguide being thermally isolated from the substrate. Any small over-etch will be uniform along the length of the via and have little effect on the thermal performance of the structure.

Fig. 4 illustrates a manufacturing process for manufacturing the structure of fig. 3. A layered structure 400 is fabricated comprising, in order, a substrate layer 406, a sacrificial layer 405, a lower cladding layer 403, a waveguide core layer 402, and an upper cladding layer 401. The upper cladding layer comprises an upper cladding layer material, e.g., a p-cladding layer material. The waveguide core layer includes a waveguide core material. The lower cladding layer comprises a lower cladding material, e.g., n-cladding material. The sacrificial layer comprises a sacrificial material. The substrate comprises a substrate material.

The layered structure 400 is then etched 4000, 4001 (e.g., using dry etching or a combination of dry and wet etching) to produce an intermediate structure 410. The first etch 4000 etches the upper cladding layer 401 to form a waveguide ridge 404 and an etched upper cladding layer 411. The intermediate structure also has vias 407 etched through the waveguide core layer 402 and the lower cladding layer 403 to leave an etched waveguide core layer 412 and an etched lower cladding layer 413. A via hole passes through the upper cladding layer, the waveguide core layer, and the lower cladding layer up to the sacrificial layer. The upper cladding layer 401 may be etched from the location of the via during the etching step 4000 (as shown), or may be etched along with the waveguide core layer 402 and the lower cladding layer 403 during the etching step 4001. If the upper cladding layer is etched only during step 4000, the sides of the etched upper cladding layer 411 may not be at the edges of the via (as shown in the figure).

The 4002 intermediate structure is then etched by using a chemically selective wet etch introduced into via 407 to produce waveguide structure 420. The wet etch preferentially etches the sacrificial material to form an etched sacrificial layer 415 such that the sacrificial material is removed from regions extending at least from the via 407 to beyond the waveguide ridge 404, leaving an air gap 408 in the regions between the lower cladding 413 and the substrate 406. The air gap causes this region to be thermally insulating. It should be noted that the waveguide structure 420 is identical to the waveguide structure shown in fig. 3.

The etch processes 4001 and 4002 may be performed separately, or the intermediate structure 410 may be created by other means and provided to the wet etch process 4002.

Figure 5 shows a thermal model of the structure shown in figure 3. Heat is applied to the waveguide ridge 304 and the substrate 306 is maintained at a constant temperature. In fig. 3, the high density dots indicate high temperatures, and the low density dots indicate low temperatures. The heat flow through the remaining sacrificial layer can be clearly seen. Thermal performance will depend on the width and thickness of the overhang, the choice of sacrificial material, and the thickness of the sacrificial material.

Typical overhang widths are 20 to 50 microns. Typical sacrificial material thicknesses are 0.25 to 2 microns. Typical thicknesses for each of the upper cladding layer, the lower cladding layer, and the waveguide core layer are 1-3 microns. The sacrificial material is typically 1-2 microns below the optical core. The adiabatic regions typically extend 10-40 microns beyond the waveguide ridge, for example 30 microns beyond the waveguide ridge. To avoid thermal effects at the overhang end, the overhang may extend at least 20 microns in the axial direction of the waveguide from critical features of the laser (e.g., the grating), at least 50 microns from these features, or at least 100 microns from these features.

The combination of materials for the sacrificial layer, the etching fluid and the cladding layer should be chosen such that the etching fluid has a good mobility (reference) for etching the sacrificial material on the cladding layer. In the case where the waveguide core is susceptible to an etching fluid, a passivating dielectric may be applied to the exposed surface of the waveguide core within the via to prevent etching of the waveguide core.

As an example, the sacrificial material used in the sacrificial layer may comprise one or more of InGaAs, AlInAs, and AlGaInAs, and the cladding layer may comprise InP. Possible etching fluids that would etch the sacrificial layer but not significantly the cladding layer include:

·H₃PO₄-H₂O₂

·H₂SO₄-H₂O₂

citric acid-H₂O₂

·HNO₃

Tartaric acid-HNO₃

Tartaric acidacid-H₂O₂

·HF-H₂O₂

The sacrificial material in the sacrificial layer remains in place on the sides of the waveguide structure and may remain in place in the regions of the device other than those that are thermally controllable. This ensures that those areas are in thermal contact with the substrate, which facilitates temperature control of those areas. Instead of leaving air gaps in the etched areas, the air gaps may be filled or partially filled with a thermally insulating material, i.e. a material that is more thermally insulating than the sacrificial material.

The sacrificial material used in the sacrificial layer may also be formed as more than one discrete layer, although in this case all of these discrete layers still together form the sacrificial material. In one arrangement, the sacrificial material may comprise an AlInAs underlayer with an InGaAs top layer. This particular arrangement has a number of advantages. Better growth morphology of subsequent layers can be achieved on InGaAs compared to AlInAs, and processing schemes comprising a combination of wet and dry etch procedures can be advantageously employed. The combination of materials allows the thermal conductivity remaining in the layer below the increased portion to be optimized. The absorption of light by the InGaAs helps to control stray (unguided) light. Alternatively, only InGaAs may be used as the sacrificial layer.

Fig. 6 shows an exemplary structure with other components. The upper and

lower cladding layers

601 and 603, the waveguide core layer 602, the waveguide ridge 604, the sacrificial layer 605, the via 607, and the substrate 606 are equivalent to those in fig. 3. The structure also includes a passivation dielectric 609, a heater resistor 610, and

ground contacts

611, 612. The passivation dielectric 609 shields the waveguide core from wet etching and electrically isolates the ridge waveguide from the heater resistor 610. The passivation dielectric may be arranged to perform one or both of these functions. Referring back to fig. 4, a passivation dielectric may be applied to intermediate structure 410 prior to introducing wet etch 4002. The heating resistor 610 provides heating to the ridge waveguide according to the thermal tuning needs of the laser. The

ground contacts

611, 612 result in a reduction of the carrier density oscillations in the waveguide core and a clamping of the fermi level, which results in suppression of shot noise and improved performance, especially in the 10MHz-100MHz range. Because p-ground contact 611 connects to the waveguide ridge better than the undercut design of the prior art, the shot noise of the illustrated structure is significantly reduced compared to an equivalent structure based on fig. 2. Other configurations of ground contacts and heaters are possible. For example, there may be a ground contact at the top of the waveguide ridge, a heater on either side of the ridge, or a ground contact in one or both gaps in the upper cladding near the ridge. The heater may be in contact with the top of the ridge, the sides of the ridge, or both.

Fig. 7 shows another exemplary structure. Upper cladding layer 701 and lower cladding layer 703, waveguide core layer 702, waveguide ridge 704, sacrificial layer 705, via 707, and substrate 706 are equivalent to those structures in fig. 3. The structure further comprises

heating resistors

709, 710,

ground contacts

711, 712 and support ridges 713. The ground contact 711 is located on top of the waveguide ridge 704 and the

heaters

709, 710 are equidistant on either side of the ridge. A passivation dielectric 714 is provided to prevent electrical contact between the heater and the semiconductor. The passivation dielectric 714 has gaps to allow contact between the ground contact 711 and the waveguide ridge 704. This arrangement provides improved phase noise relative to the arrangement of figure 6. The support ridges 713 improve the mechanical strength of the overhang in a manner similar to "C-beams" or "parallel flange channels" used in the mechanical arts. The upper cladding 701 supporting the ridges 713 may be thickened to improve mechanical integrity.

The additional features of fig. 6 and 7 may be combined in any suitable arrangement, or with other features mentioned in the disclosure but not shown in the drawings. For example, the structure may be provided with an arrangement with

ground contacts

611, 612 and supporting ridges 713, or with

heating resistors

709, 710 and ground contacts 712 without an arrangement of supporting ridges 713.

A similar thermal isolation structure can be applied to a buried heterostructure laser as shown in fig. 8. Fig. 8 shows a waveguide structure comprising upper and lower cladding layers 801, 803, waveguide core layer 802, waveguide 804, isolation region 808, sacrificial layer 805, via 807 and substrate 806. The substrate 806, vias 807 and sacrificial layer 805 are identical to those of fig. 6 and 7. The waveguide 804 and the isolation region 808 are equivalent to the structure 115 and the isolation region 116 of fig. 1B. The

ground contacts

811, 812 are shown in an arrangement corresponding to fig. 6, but may also be the same arrangement as fig. 7. The heaters may be applied at any suitable location around the waveguide 804, such as on either side of the waveguide, as shown by

heaters

809, 810. A passivating dielectric is applied to prevent electrical contact between the heater 814 and the underlying components. To provide sufficient thermal isolation, the undercut should extend at least beyond the waveguide core, i.e., beyond the waveguide structure 804.

The waveguide structure disclosed above can be used with any waveguide having a thermally controllable portion. For example, in a Distributed Bragg Reflector (DBR) laser, a waveguide structure may be used for the rear DBR section and/or the phase control section to provide improved thermal control of these sections.

Claims

1. A waveguide structure comprising a waveguide having a thermally controllable portion, the waveguide structure comprising a plurality of layers and a ground contact, the layers comprising in sequence:

a substrate, a first electrode and a second electrode,

a sacrificial layer is formed on the substrate,

a lower cladding layer,

a waveguide core layer, and

an upper cladding layer including a waveguide ridge;

wherein:

the lower cladding, waveguide core layer and upper cladding layer forming the waveguide, the waveguide having a waveguide core;

the region of the waveguide core layer below the waveguide ridge forms the waveguide core;

the waveguide structure having a continuous via passing through the upper cladding layer, the waveguide core layer and the lower cladding layer and extending parallel to the waveguide core along substantially the entire length of the thermally controllable portion on only one side of the waveguide ridge;

the sacrificial layer comprises an adiabatic region extending from at least the via beyond the waveguide core along an entire length of the thermally controllable section, wherein the sacrificial layer comprises a sacrificial material outside the adiabatic region, and an adiabatic gap or an adiabatic material separating the lower cladding and the substrate within the adiabatic region; and is

The ground contact is in electrical contact with the upper cladding on a side of the waveguide core opposite the via; or

The ground contact is in electrical contact with the upper cladding adjacent the waveguide core.

2. The waveguide structure of claim 1, wherein the adiabatic region extends from the via to at least 5 microns beyond the waveguide core.

3. The waveguide structure of claim 1, comprising a passivating dielectric applied to an exposed surface of the waveguide core layer.

4. The waveguide structure of claim 1, comprising a heater in thermal contact with the upper cladding adjacent the waveguide core.

5. The waveguide structure of claim 1, comprising two heaters positioned on either side of the waveguide core.

6. The waveguide structure of claim 4, comprising a passivating dielectric applied between the upper cladding layer and the heater, wherein the passivating dielectric is configured to electrically insulate the waveguide ridge from the heater.

7. The waveguide structure of claim 4, wherein the heater is a heating resistor.

8. The waveguide structure of claim 1, wherein the sacrificial layer is between 0.5 and 2 microns thick.

9. A waveguide structure according to claim 1, wherein the adiabatic regions have a width of 20 to 40 microns, the width being measured in a direction perpendicular to the waveguide core and in the plane of the sacrificial layer.

10. The waveguide structure of claim 1, wherein the sacrificial material comprises any one or more of:

the crystal structure of indium, gallium, arsenic and InGaAs,

aluminum indium arsenic AlInAs, and

AlGaInAs.

11. The waveguide structure of claim 1, wherein the sacrificial material comprises a lower layer of AlInAs and an upper layer of InGaAs.

12. The waveguide structure of claim 1, wherein the upper cladding layer includes a waveguide ridge positioned adjacent to the waveguide core.

13. The waveguide structure of claim 12, comprising a support ridge positioned between the waveguide ridge and the via, the support ridge extending parallel to the waveguide ridge along substantially an entire length of the thermally controllable portion.

14. The waveguide structure of claim 1, wherein the waveguide structure further comprises an isolation region on either side of the waveguide, the isolation region insulating the waveguide from other portions of the upper cladding layer and waveguide core layer.

15. A tunable laser comprising the waveguide structure of claim 1.

16. The laser according to claim 15, wherein the thermally controllable portion of the waveguide forms part of a distributed bragg reflector.

17. The laser of claim 15, wherein the thermally controllable portion of the waveguide forms part of a phase controller in a laser cavity.

18. A method of manufacturing a thermally controlled waveguide, the method comprising:

providing a waveguide structure comprising a plurality of layers and a ground contact, the layers comprising in order:

a substrate, a first electrode and a second electrode,

a sacrificial layer is formed on the substrate,

a lower cladding layer,

a waveguide core layer, and

an upper cladding layer including a waveguide ridge;

the waveguide structure having a continuous via that passes through the upper cladding layer, the waveguide core layer and the lower cladding layer and extends parallel to the waveguide ridge along the entire length of the thermally controllable section on only one side of the waveguide ridge;

the waveguide structure is arranged such that:

The ground contact is in electrical contact with the upper cladding adjacent the waveguide core;

providing a wet etch to the sacrificial layer through the via to remove material from at least an adiabatic region in the sacrificial layer extending along an entire length of the thermally controllable portion at least from the via to beyond the waveguide ridge to create a gap in the adiabatic region separating the lower cladding layer and substrate, wherein the wet etch etches material of the sacrificial layer and does not etch material of the substrate and lower cladding layer.

19. The method of claim 18, wherein the waveguide structure further comprises a passivating dielectric applied to an exposed surface of the waveguide core layer.

20. The method of claim 18, wherein the wet etch is any one of:

H₃PO₄-H₂O₂，

H₂SO₄-H₂O₂，

citric acid-H₂O₂，

HNO₃，

Tartaric acid-HNO₃，

Tartaric acid-H₂O₂And an

HF-H₂O₂。

21. The method of claim 18, wherein the sacrificial layer comprises any one or more of:

the crystal structure of indium, gallium, arsenic and InGaAs,

aluminum indium arsenic AlInAs, and

AlGaInAs.

22. The method of claim 18, wherein the sacrificial layer comprises an AlInAs layer and an InGaAs layer.

23. The method of claim 18, wherein the step of providing the waveguide structure comprises:

etching the upper cladding layer to form a waveguide ridge;

etching the upper cladding layer, the waveguide core layer and the lower cladding layer to form the via.

24. The method of claim 23, wherein the steps of etching the upper cladding layer and etching the upper cladding layer, waveguide core layer, and lower cladding layer comprise dry etching or a combination of dry etching and wet etching.

25. The method of claim 23, prior to the steps of etching the upper cladding layer and etching the upper cladding layer, waveguide core layer, and lower cladding layer, the method comprising:

manufacturing a layered structure comprising in sequence:

the substrate is provided with a plurality of grooves,

the sacrificial layer is formed on the substrate,

the lower cladding layer is provided with a plurality of layers,

the waveguide core layer, and

the upper cladding layer.